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<div class="titlepage"><div><div><h4 class="title">
<a name="beast.ref.boost__beast__http__async_read_header"></a><a class="link" href="boost__beast__http__async_read_header.html" title="http::async_read_header">http::async_read_header</a>
</h4></div></div></div>
<p>
        <a class="indexterm" name="idm44861243819600"></a>
      </p>
<p>
        Read a complete message header asynchronously from a stream using a parser.
      </p>
<h5>
<a name="beast.ref.boost__beast__http__async_read_header.h0"></a>
        <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.synopsis"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.synopsis">Synopsis</a>
      </h5>
<p>
        Defined in header <code class="literal">&lt;<a href="../../../../../../boost/beast/http/read.hpp" target="_top">boost/beast/http/read.hpp</a>&gt;</code>
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">class</span> <a href="../../../../../../doc/html/boost_asio/reference/AsyncReadStream.html" target="_top"><span class="emphasis"><em>AsyncReadStream</em></span></a><span class="special">,</span>
    <span class="keyword">class</span> <a class="link" href="../concepts/DynamicBuffer.html" title="DynamicBuffer"><span class="emphasis"><em>DynamicBuffer</em></span></a><span class="special">,</span>
    <span class="keyword">bool</span> <span class="identifier">isRequest</span><span class="special">,</span>
    <span class="keyword">class</span> <a href="../../../../../../doc/html/boost_asio/reference/ReadHandler.html" target="_top"><span class="emphasis"><em>ReadHandler</em></span></a> <span class="special">=</span> <span class="identifier">net</span><span class="special">::</span><span class="identifier">default_completion_token_t</span><span class="special">&lt;</span>            <span class="identifier">executor_type</span><span class="special">&lt;</span><span class="identifier">AsyncReadStream</span><span class="special">&gt;&gt;&gt;</span>
<a href="../../../../../../doc/html/boost_asio/reference/asynchronous_operations.html#boost_asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type" target="_top"><span class="emphasis"><em>DEDUCED</em></span></a>
<span class="identifier">async_read_header</span><span class="special">(</span>
    <span class="identifier">AsyncReadStream</span><span class="special">&amp;</span> <span class="identifier">stream</span><span class="special">,</span>
    <span class="identifier">DynamicBuffer</span><span class="special">&amp;</span> <span class="identifier">buffer</span><span class="special">,</span>
    <span class="identifier">basic_parser</span><span class="special">&lt;</span> <span class="identifier">isRequest</span> <span class="special">&gt;&amp;</span> <span class="identifier">parser</span><span class="special">,</span>
    <span class="identifier">ReadHandler</span><span class="special">&amp;&amp;</span> <span class="identifier">handler</span> <span class="special">=</span> <span class="identifier">net</span><span class="special">::</span><span class="identifier">default_completion_token_t</span><span class="special">&lt;</span> <a class="link" href="boost__beast__executor_type.html" title="executor_type">executor_type</a><span class="special">&lt;</span> <span class="identifier">AsyncReadStream</span> <span class="special">&gt;&gt;{});</span>
</pre>
<h5>
<a name="beast.ref.boost__beast__http__async_read_header.h1"></a>
        <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.description"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.description">Description</a>
      </h5>
<p>
        This function is used to asynchronously read a complete message header from
        a stream into an instance of <a class="link" href="boost__beast__http__basic_parser.html" title="http::basic_parser"><code class="computeroutput"><span class="identifier">basic_parser</span></code></a>. The function call
        always returns immediately. The asynchronous operation will continue until
        one of the following conditions is true:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            <a class="link" href="boost__beast__http__basic_parser/is_header_done.html" title="http::basic_parser::is_header_done"><code class="computeroutput"><span class="identifier">basic_parser</span><span class="special">::</span><span class="identifier">is_header_done</span></code></a> returns <code class="computeroutput"><span class="keyword">true</span></code>
          </li>
<li class="listitem">
            An error occurs.
          </li>
</ul></div>
<p>
        This operation is implemented in terms of zero or more calls to the next
        layer's <code class="computeroutput"><span class="identifier">async_read_some</span></code> function,
        and is known as a <span class="emphasis"><em>composed operation</em></span>. The program must
        ensure that the stream performs no other reads until this operation completes.
        The implementation may read additional bytes from the stream that lie past
        the end of the message being read. These additional bytes are stored in the
        dynamic buffer, which must be preserved for subsequent reads. If the end
        of file error is received while reading from the stream, then the error returned
        from this function will be:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            <a class="link" href="boost__beast__http__error.html" title="http::error"><code class="computeroutput"><span class="identifier">error</span><span class="special">::</span><span class="identifier">end_of_stream</span></code></a>
            if no bytes were parsed, or
          </li>
<li class="listitem">
            <a class="link" href="boost__beast__http__error.html" title="http::error"><code class="computeroutput"><span class="identifier">error</span><span class="special">::</span><span class="identifier">partial_message</span></code></a>
            if any bytes were parsed but the message was incomplete, otherwise:
          </li>
<li class="listitem">
            A successful result. The next attempt to read will return <a class="link" href="boost__beast__http__error.html" title="http::error"><code class="computeroutput"><span class="identifier">error</span><span class="special">::</span><span class="identifier">end_of_stream</span></code></a>
          </li>
</ul></div>
<h5>
<a name="beast.ref.boost__beast__http__async_read_header.h2"></a>
        <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.parameters"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.parameters">Parameters</a>
      </h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Name
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">stream</span></code>
                </p>
              </td>
<td>
                <p>
                  The stream from which the data is to be read. The type must meet
                  the <span class="emphasis"><em>AsyncReadStream</em></span> requirements.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">buffer</span></code>
                </p>
              </td>
<td>
                <p>
                  Storage for additional bytes read by the implementation from the
                  stream. This is both an input and an output parameter; on entry,
                  the parser will be presented with any remaining data in the dynamic
                  buffer's readable bytes sequence first. The type must meet the
                  <span class="emphasis"><em>DynamicBuffer</em></span> requirements. The object must
                  remain valid at least until the handler is called; ownership is
                  not transferred.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">parser</span></code>
                </p>
              </td>
<td>
                <p>
                  The parser to use. The object must remain valid at least until
                  the handler is called; ownership is not transferred.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <code class="computeroutput"><span class="identifier">handler</span></code>
                </p>
              </td>
<td>
                <p>
                  The completion handler to invoke when the operation completes.
                  The implementation takes ownership of the handler by performing
                  a decay-copy. The equivalent function signature of the handler
                  must be:
                </p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span>
    <span class="identifier">error_code</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">error</span><span class="special">,</span>        <span class="comment">// result of operation</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span>   <span class="comment">// the total number of bytes transferred from the stream</span>
<span class="special">);</span>
</pre>
                <p>
                  Regardless of whether the asynchronous operation completes immediately
                  or not, the handler will not be invoked from within this function.
                  Invocation of the handler will be performed in a manner equivalent
                  to using <a class="link" href="boost__beast__http__verb.html" title="http::verb"><code class="computeroutput"><span class="identifier">net</span><span class="special">::</span><span class="identifier">post</span></code></a>.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<h5>
<a name="beast.ref.boost__beast__http__async_read_header.h3"></a>
        <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.remarks"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.remarks">Remarks</a>
      </h5>
<p>
        The completion handler will receive as a parameter the total number of bytes
        transferred from the stream. This may be zero for the case where there is
        sufficient pre-existing message data in the dynamic buffer. The implementation
        will call <a class="link" href="boost__beast__http__basic_parser/eager.html" title="http::basic_parser::eager"><code class="computeroutput"><span class="identifier">basic_parser</span><span class="special">::</span><span class="identifier">eager</span></code></a> with the value <code class="computeroutput"><span class="keyword">false</span></code> on the parser passed in.
      </p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright © 2016-2019 Vinnie
      Falco<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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